Chemical neurostimulation using pulse code modulation (PCM) microfluidic chips

A microfluidic array enabling generation of identical biochemical stimulating signals to trapped biological cells for single-cell dynamics

Microfluidic-based analyses of single-cell dynamics in response to dynamic biochemical signals are emerging as pivotal approaches for investigating the effects of extracellular microenvironmental biochemical factors on cellular structure, function, and behavior. However, current devices often fail to consistently apply identical dynamic biochemical signals to trapped cells. In this study, we introduce a novel radially distributed single-cell trapping microfluidic array, designed to quantitatively and consistently apply identical biochemical stimulating signals to each trapped cell. Numerical simulations were employed to optimize microchannel geometry, enhancing trapping efficiency while minimizing signal distortion. Experimental validation demonstrated the trapping success rate and the single-cell trapping efficiency exceeding 99% and 85%, respectively. The microarray's capability to deliver identical dynamic biochemical stimulating signals, with various waveforms, to each unit was confirmed through fluorescein transport tests. Furthermore, we examined the intracellular calcium dynamics of U-2 OS human osteosarcoma cells in response to dynamic ATP signals, observing both single-peak calcium responses and calcium oscillations, which were modelled by a second-order system with a natural frequency of 1.6 mHz. Overall, our proposed microfluidic array offers a robust and valuable framework for advancing the understanding of single-cell dynamics.

Use of an invertebrate animal model (Aplysia californica) to develop novel neural interfaces for neuromodulation

New tools for monitoring and manipulating neural activity have been developed with steadily improving functionality, specificity, and reliability, which are critical both for mapping neural circuits and treating neurological diseases. This review focuses on the use of an invertebrate animal, the marine mollusk Aplysia californica, in the development of novel neurotechniques. We review the basic physiological properties of Aplysia neurons and discuss the specific aspects that make it advantageous for developing novel neural interfaces: First, Aplysia nerves consist only of unmyelinated axons with various diameters, providing a particularly useful model of the unmyelinated C fibers in vertebrates that are known to carry important sensory information, including those that signal pain. Second, Aplysia's neural tissues can last for a long period in an ex vivo experimental setup. This allows comprehensive tests such as the exploration of parameter space on the same nerve to avoid variability between animals and minimize animal use. Third, nerves in large Aplysia can be many centimeters in length, making it possible to easily discriminate axons with different diameters based on their conduction velocities. Aplysia nerves are a particularly good approximation of the unmyelinated C fibers, which are hard to stimulate, record, and differentiate from other nerve fibers in vertebrate animal models using epineural electrodes. Fourth, neurons in Aplysia are large, uniquely identifiable, and electrically compact. For decades, researchers have used Aplysia for the development of many novel neurotechnologies. Examples include high-frequency alternating current (HFAC), focused ultrasound (FUS), optical neural stimulation, recording, and inhibition, microelectrode arrays, diamond electrodes, carbon fiber microelectrodes, microscopic magnetic stimulation and magnetic resonance electrical impedance tomography (MREIT). We also review a specific example that illustrates the power of Aplysia for accelerating technology development: selective infrared neural inhibition of small-diameter unmyelinated axons, which may lead to a translationally useful treatment in the future. Generally, Aplysia is suitable for testing modalities whose mechanism involves basic biophysics that is likely to be similar across species. As a tractable experimental system, Aplysia californica can help the rapid development of novel neuromodulation technologies.

A review on microfluidics manipulation of the extracellular chemical microenvironment and its emerging application to cell analysis

Spatiotemporal manipulation of extracellular chemical environments with simultaneous monitoring of cellular responses plays an essential role in exploring fundamental biological processes and expands our understanding of underlying mechanisms. Despite the rapid progress and promising successes in manipulation strategies, many challenges remain due to the small size of cells and the rapid diffusion of chemical molecules. Fortunately, emerging microfluidic technology has become a powerful approach for precisely controlling the extracellular chemical microenvironment, which benefits from its integration capacity, automation, and high-throughput capability, as well as its high resolution down to submicron. Here, we summarize recent advances in microfluidics manipulation of the extracellular chemical microenvironment, including the following aspects: i) Spatial manipulation of chemical microenvironments realized by convection flow-, diffusion-, and droplet-based microfluidics, and surface chemical modification; ii) Temporal manipulation of chemical microenvironments enabled by flow switching/shifting, moving/flowing cells across laminar flows, integrated microvalves/pumps, and droplet manipulation; iii) Spatiotemporal manipulation of chemical microenvironments implemented by a coupling strategy and open-space microfluidics; and iv) High-throughput manipulation of chemical microenvironments. Finally, we briefly present typical applications of the above-mentioned technical advances in cell-based analyses including cell migration, cell signaling, cell differentiation, multicellular analysis, and drug screening. We further discuss the future improvement of microfluidics manipulation of extracellular chemical microenvironments to fulfill the needs of biological and biomedical research and applications.

Transport of dynamic biochemical signals in a microfluidic single cell trapping channel with varying cross-sections

Dynamic biochemical signal control in vitro is important in the study of cellular responses to dynamic biochemical stimuli in microenvironment in vivo. To this end, we designed a microfluidic single cell trapping channel with varying cross-sections. In this work, we analyzed the transport of dynamic biochemical signals in steady and non-reversing pulsatile flows in such a microchannel. By numerically solving the 2D time-dependent Taylor-Aris dispersion equation, we studied the transport mechanism of different signals with varying parameters. The amplitude spectrum in steady flow shows that the trapping microchannel acts as a low-pass filter due to the longitudinal dispersion. The input signal can be modulated nonlinearly by the pulsatile flow. In addition, the nonlinear modulation effects are affected by the pulsatile flow frequency, the pulsatile flow amplitude and the average flow rate. When the flow frequency is much smaller or larger than that of the biochemical signal, the signal can be transmitted more efficiently. Besides, smaller pulsatile flow amplitude and larger average flow rate can decrease the nonlinear modulation and promote the signal transmission. These results demonstrate that in order to accurately load a desired dynamic biochemical signal to the trapped cell to probe the cellular dynamic response to the dynamic biochemical stimulus, the transport mechanism of the signals in the microchannel should be carefully considered.

Concentration gradient generation methods based on microfluidic systems

Various concentration gradient generation methods based on microfluidic systems are summarized in this paper.

Combinatorial generation of droplets by controlled assembly and coalescence

We describe a microfluidic system for generating a sequence of liquid droplets of multiple concentrations in a single experimental condition. The series of final droplets has the combination of the compositions varying periodically, with polydispersity of the size less than 8%. By utilizing the design of the microchannel geometry and the passive control of three immiscible fluids (oil, water, and air) including generation, breakup, separation and coalescence of droplets, we can change the system to generate diverse sets of combination of materials. The device can be used for testing different concentration of materials in picoliter volumes and developing a new way to deliver dynamic signals of chemicals with microfluidics.

Extracellularly identifying motor neurons for a muscle motor pool in Aplysia californica

In animals with large identified neurons (e.g. mollusks), analysis of motor pools is done using intracellular techniques. Recently, we developed a technique to extracellularly stimulate and record individual neurons in Aplysia californica. We now describe a protocol for using this technique to uniquely identify and characterize motor neurons within a motor pool. This extracellular technique has advantages. First, extracellular electrodes can stimulate and record neurons through the sheath, so it does not need to be removed. Thus, neurons will be healthier in extracellular experiments than in intracellular ones. Second, if ganglia are rotated by appropriate pinning of the sheath, extracellular electrodes can access neurons on both sides of the ganglion, which makes it easier and more efficient to identify multiple neurons in the same preparation. Third, extracellular electrodes do not need to penetrate cells, and thus can be easily moved back and forth among neurons, causing less damage to them. This is especially useful when one tries to record multiple neurons during repeating motor patterns that may only persist for minutes. Fourth, extracellular electrodes are more flexible than intracellular ones during muscle movements. Intracellular electrodes may pull out and damage neurons during muscle contractions. In contrast, since extracellular electrodes are gently pressed onto the sheath above neurons, they usually stay above the same neuron during muscle contractions, and thus can be used in more intact preparations. To uniquely identify motor neurons for a motor pool (in particular, the I1/I3 muscle in Aplysia) using extracellular electrodes, one can use features that do not require intracellular measurements as criteria: soma size and location, axonal projection, and muscle innervation. For the particular motor pool used to illustrate the technique, we recorded from buccal nerves 2 and 3 to measure axonal projections, and measured the contraction forces of the I1/I3 muscle to determine the pattern of muscle innervation for the individual motor neurons. We demonstrate the complete process of first identifying motor neurons using muscle innervation, then characterizing their timing during motor patterns, creating a simplified diagnostic method for rapid identification. The simplified and more rapid diagnostic method is superior for more intact preparations, e.g. in the suspended buccal mass preparation or in vivo. This process can also be applied in other motor pools in Aplysia or in other animal systems.

Temporal gradients in microfluidic systems to probe cellular dynamics: a review

Microfluidic devices have found a unique place in cellular studies due to the ease of fabrication, their ability to provide long-term culture, or the seamless integration of downstream measurements into the devices. The accurate and precise control of fluid flows also allows unique stimulant profiles to be applied to cells that have been difficult to perform with conventional devices. In this review, we describe and provide examples of microfluidic systems that have been used to generate temporal gradients of stimulants, such as waveforms or pulses, and how these profiles have been used to produce biological insights into mammalian cells that are not typically revealed under static concentration gradients. We also discuss the inherent analytical challenges associated with producing and maintaining temporal gradients in these devices.